Electronic equipment such as radar, instrumentation and computer electronics are often contained in sealed shielded enclosures. This equipment requires shielding because electromagnetic energy in the radio frequency range interferes with the equipment's internal electronics. An effectively sealed and shielded enclosure prevents electromagnetic energy within it from inadvertently leaking out and thereby interfering with the proper operation of electromagnetically sensitive electronic equipment exterior to it and conversely prevents electromagnetic energy exterior to it from inadvertently entering it and causing problems with its proper operation. If the shielded enclosure is theoretically perfect, the electromagnetic isolation between the interior and exterior is perfect. However, shielded enclosures that house electronic equipment are less than perfect. Practical shielded enclosures require external access into the enclosure for such reasons as input and output connection, control devices, and for access covers, ports, doors and other types of access which are required for repair or adjustment to the internal electronic components. To maintain maximum shielding effectiveness, it is essential that enclosure accesses be as electromagentically leak-free as possible. Where input/output connectors or other similar type devices penetrate through enclosure walls, welding or other permanent sealing shielding means can many times be employed to prevent electromagnetic leakage, but for enclosure accesses that must be removable, such as covers, ports and doors, an electromagnetic gasket shield is required. There are many requirements that must be considered in order to provide effective electromagnetic gasket shielding; this is especially true for critical shielding applications.
Critical electromagnetic gasketing shielding applications, such as are found in military and aerospace applications and others, make demands on electromagnetic gasketing shielding materials that heretofore have not been completely met. Problems such as cracking or crazing of gasketing shielding materials when exposed to temperature extremes and high vibration, flaking of metallic fillers, critical torquing requirements when gasketed flanges are bolted together, solvent attack, chemical and electrochemical induced corrosion, high outgassing owing to residual unreacted components, and heavy weight are a few of the undesirable characteristics that cause problems which limit the usefulness of existing materials and degrade electromagnetic gasketing shielding effectiveness.
Important requirements for effective electromagnetic gasket shielding for critical applications are characterized by the electrical properties of high electrical conductivity and low resistivity to ensure maximum transfer of electromagnetic energy across the gasketed interfaces of the enclosure to minimize electromagnetic leakage. Maximum interfacial transfer of electromagnetic energy results in high shielding effectiveness.
Mechanical sealing properties are also important. An effective mechanical seal as well as effective electromagnetic shield prevents dust, dirt or other potentially damaging external environmental elements from entering into the shielded enclosure while maintaining the integrity of the internal environment of the enclosure. An effective mechanical/electromagnetic seal conforms easily to irregular as well as highly machined mating surfaces, does not deteriorate owing to age, is resistant to solvent attack, chemical or electrochemical induced corrosion, compression forces, functions effectively and is undamaged throughout a wide temperature range, remains leak-free under both high positive and negative pressures, does not require critical torquing to maintain its gasketing and shielding electromagnetic integrity, has low gas diffusion rates when compressed and densified, and has no outgassing. Also of importance is high compression and shear strength, flexibility and resiliency, light weight, and, if metal filled or plated, no cracking or flaking under the stresses of gasket shielding applications.
There are many good electromagnetic shielding gasketing materials commercially available which are utilized in critical applications, one of the best being conductive elastomeric gasketing. Conductive elastomeric gasketing utilizes silicone or fluorosilicone as the elastomeric binder and silver, silver-copper alloy, or silver-aluminum particles as the conductive filler. The best conductive elastomeric gasketing provides 70 to 120 DB shielding effectiveness in the electromagnetic frequency range of 200 kilohertz to 10 gigahertz. However, because of the many critical electromagnetic gasket shielding applications that require most or all of the above properties, to date it has not been possible with conductive elastomeric electromagnetic shielding or with other previous materials or means to produce electromagnetic gasketing shielding that meets all of the above requirements for critical applications. This invention provides a process and composition for the effective gasketing and electromagnetic energy shielding for critical gasketing and shielding applications through this use of metal-plated microporous PTFE.
Polytetrafluoroethylene (PTFE) seals and gaskets per se are known in the art. West German 3,339,018 describes a seal made from extruded and oriented microporous PTFE which has a greater height than width to compensate for the unevenness of flange surfaces to be sealed. Only small flange forces are needed for a predetermined leak pressure. No fillers are mentioned or application to electromagnetic gasketing. U.S. Pat. No. 4,147,824 shows plastic seals for use in valves which control reactive and aggressive chemicals made from porous PTFE covered with smooth dense PTFE, which layers are sintered together. The porous PTFE is prepared from filled PTFE from which the filler has been dissolved by a solvent to leave a porous-structure. The material is used for annular pressure seals. Another type of sealing material containing PTFE is disclosed in U.S. Pat. No. 4,042,747 which comprises particulate material interconnected and entrapped by fibrillated PTFE. Inorganic particulates were utilized in the seals.